https://www.siliconvalley.com/2020/12/07/coronavirus-how-you-can-opt-in-to-californias-new-exposure-notification-app/

So, you’ve heard California has a new app that allows residents to be notified when they’ve come in contact with somebody infected with COVID-19 and want to learn more about it. Well, you’ve come to the right place.

First of all, the app — dubbed CA Notify — goes live Thursday. It was created in partnership with Apple and Google, and it allows users of those smartphones who opt in to receive notifications if they have been in close proximity with another user of the app who has tested positive. The more people who sign up, the more effective the app is. Other states that have rolled out similar programs have had difficulty convincing large swaths of the population to opt in.